1. Field of the Invention
The present invention relates to imager cells and, more particularly, to a bandgap tuned vertical color imager cell.
2. Description of the Related Art
A standard imager cell is a device that utilizes a photodiode to convert light energy into photo-generated electrons, and a number of transistors to control the operation of the photodiode and sense the number of photo-generated electrons that were produced by the light energy.
A color imager cell is a cell that produces photo information for more than one color, typically red, green, and blue. Digital cameras utilize millions of color imager cells that capture images based on the amount of reflected red, green, and blue light energy that strikes each imager cell.
One approach to implementing a color imager cell is to utilize three standard imager cells, and vary the depth of the sense point. Blue photons are high-energy photons that are absorbed at the surface of the photodiode, while green photons have less energy and are absorbed at a first depth that lies below the surface of the photodiode. Red photons have even less energy and are absorbed at a second depth that lies below the first depth.
Thus, by sensing the number of photo-generated electrons at the surface of a first cell, at the first depth in a second cell, and at the second depth in a third cell, blue, green, and red photo information, respectively, can be obtained from the three standared imager cells. One drawback of this approach, however, is that a color imager cell is three times the size of a standard imager cell.
Another approach to implementing a color imager cell that addresses the size problems of the three-cell approach is a vertical color imager cell. A vertical color imager cell is an imager cell that collects photo-generated electrons that represent different colors, such as red, green, and blue, at different depths in the same photodiode.
In an imager cell, the transistors required to operate the photodiode and sense the number of photo-generated electrons utilize a relatively small amount of space with respect to the amount of space utilized by the photodiode. Thus, one of the advantages of a vertical color imager cell is that since the cell collects multiple colors with the same photodiode, a vertical color imager cell is only slightly larger than a standard imager cell.
U.S. Patent Application Publication US 2002/0058353 A 1 published on May 16, 2002 describes a vertical color imager cell. FIG. 1 shows a combined cross-sectional and schematic diagram that illustrates a prior art color imager cell 100. Cell 100 is substantially the same as the cell shown in FIG. 2A of the ""353 published application.
As shown in FIG. 1, imager cell 100 includes a first pxe2x88x92 region 110, a first n+ region 112 that contacts pxe2x88x92 region 110, and a first depletion region 114 that is formed at the junction between regions 110 and 112. Imager cell 100 also includes a second pxe2x88x92 region 120 that contacts n+ region 112, a second n+ region 122 that contacts pxe2x88x92 region 120, and a second depletion region 124 that is formed at the junction between regions 120 and 122. In addition, imager cell 100 further includes a third p-type region 130 that contacts n+ region 122, a third n+ region 132 that contacts pxe2x88x92 region 130, and a third depletion region 134 that is formed at the junction between regions 130 and 132.
In operation, as shown in FIG. 1, pxe2x88x92 regions 110, 120, and 130 are connected to ground. In addition, n+ regions 112, 122, and 132 are connected to first, second, and third reset transistors 150, 152, and 154, respectively. Prior to collecting photo information, reset transistors 150, 152, and 154 are pulsed on which, in turn, places a positive potential on n+ regions 112, 122, and 132.
The positive potential reverse biases the pn junction of regions 110 and 112, thereby forming a red collecting photodiode, and the pn junction of regions 120 and 122, thereby forming a green collecting photodiode. The positive potential also reverse biases the pn junction of regions 130 and 132, thereby forming a blue collecting photodiode.
Once the positive potentials have been placed on n+ regions 112, 122, and 132, light energy, in the form of photons, is collected by the red, green, and blue photodiodes. The red photons are absorbed by the red photodiode which, in turn, forms a number of red electron-hole pairs, while the green photons are absorbed by the green photodiode which, in turn, forms a number of green electron-hole pairs. Similarly, the blue photons are absorbed by the blue photodiode which, in turn, forms a number of blue electron-hole pairs.
The red electrons from the electron-hole pairs that are formed in depletion region 114 move under the influence of the electric field towards n+ region 112, where each additional electron collected by n+ region 112 reduces the positive potential that was placed on n+ region 112 by reset transistor 150. On the other hand, the holes formed in depletion region 114 move under the influence of the electric field towards pxe2x88x92 region 110.
In addition, the electrons, which are from the electron-hole pairs that are formed in pxe2x88x92 region 110 within a diffusion length of depletion region 114, diffuse to depletion region 114 and are swept to n+ region 112 under the influence of the electric field. Further, the electrons that are formed in n+ region 112 remain in n+ region 112.
Similarly, the green electrons from the electron-hole pairs that are formed in depletion region 124 move under the influence of the electric field towards n+ region 122, where each additional electron collected by n+ region 122 reduces the positive potential that was placed on n+ region 122 by reset transistor 152. On the other hand, the holes formed in depletion region 124 move under the influence of the electric field towards pxe2x88x92 region 120.
In addition, the electrons, which are from the electron-hole pairs that are formed in pxe2x88x92 region 120 within a diffusion length of depletion region 124, diffuse to depletion region 124 and are swept to n+ region 122 under the influence of the electric field. Further, the electrons that are formed in n+ region 122 remain in n+ region 122.
As with the red and green electrons, the blue electrons from the electron-hole pairs that are formed in depletion region 134 move under the influence of the electric field towards n+ region 132, where each additional electron collected by n+ region 132 reduces the positive potential that was placed on n+ region 132 by reset transistor 154. On the other hand, the holes formed in depletion region 134 move under the influence of the electric field towards pxe2x88x92 region 130.
In addition, the electrons, which are from the electron-hole pairs that are formed in pxe2x88x92 region 130 within a diffusion length of depletion region 134, diffuse to depletion region 134 and are swept to n+ region 132 under the influence of the electric field. Further, the electrons that are formed in n+ region 132 remain in n+ region 132.
After the red, green, and blue photodiodes have collected light energy for a period of time, known as the integration period, sense circuitry associated with the photodiodes detects the change in potential on n+ regions 112, 122, and 132. Specifically, in addition to a reset transistor, each photodiode also has an associated source follower transistor SF and a row select transistor RS.
The change in potentials on an n+ region is present on the gate of the associated source follower transistor SF, while the source of the source follower transistor SF is one diode drop below the potential. Thus, when the gate of the row select transistor RS is pulsed, an output potential equal to the photodiode potential less a diode drop is output to a sense cell to determine the output potential. Once the change in positive potential has been determined, the photodiodes are reset and the process is repeated.
One problem with imager cell 100 is that the red, green, and blue photodiodes of cell 100 do not produce an equal number of photo-generated electrons when exposed to a white light source. The blue photodiode produces the largest number, with the green photodiode next and the red photodiode producing the smallest number of photo-generated electrons. The difference in the numbers of electrons must then be compensated for to produce an equal response.
The present invention provides a vertical imager cell that utilizes a combination of materials to adjust the band gaps and, in turn, adjust the photon absorption rate of the color photodiodes used in an imager cell. By adjusting the photon absorption rate, the numbers of electrons collected by the photodiodes, and thereby the characteristics of the imager, can be adjusted.
An imager cell in accordance with the present invention includes a first layer of material, and a second layer of material that is formed on the first layer of material. In addition, the imager cell also includes a third layer of material that is formed on the second layer of material, and a fourth layer of material that is formed on the third layer of material. Further, a top layer of material is formed over the fourth layer of material. The top layer of material is different from the fourth layer of material.
The imager cell additionally includes a first region of a first conductivity type that is formed in the first layer of material and a lower portion of the second layer of material. The imager cell further includes a first region of a second conductivity type that is formed in an upper portion of the second layer of material to contact the first region of the first conductivity type. In addition, a first depletion region is formed between the first regions of the first and second conductivity types.
Further, the imager cell includes a second region of the first conductivity type that is formed in the third layer of material and the lower portion of fourth layer of material to contact the first region of the second conductivity type. In addition, the imager cell includes a second region of the second conductivity type that is formed in an upper portion of the fourth layer of material to contact the second region of the first conductivity type. Further, a second depletion region is formed between the second regions of the first and second conductivity types.
The imager cell also includes a third region of the first conductivity type that is formed in the top layer of material. In addition, the imager cell includes a third region of the second conductivity type that is formed in a surface of the top layer of material to contact the third region of the first conductivity type. Further, a third depletion region is formed between the third regions of the first and second conductivity types.
The present invention also includes a method of forming an imager cell that includes the steps of forming a first layer of material, and forming a second layer of material, which has a first conductivity type, on the first layer of material. The method also includes the steps of doping a top portion of the second layer of material to have a second conductivity type, and forming a third layer of material on the second layer of material.
The method additionally includes the steps of forming a fourth layer of material of the first conductivity type on the third layer of material, and doping a top portion of the fourth layer of material to have the second conductivity type. Further, the method includes the steps of forming a top layer of material of the first conductivity type over the fourth layer of material, and doping a top portion of the top layer of material to have the second conductivity type.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.